In conventional injection blow molding apparatus, the blowing of the parison in the blow mold and the cooling of the blown article sufficiently to make it stiff enough for removal from the mold, takes more time than the operations at the injection station and the stripper station. In order to increase the production of such a blow molding apparatus, it is necessary to shorten the time required for the blowing operation.
This invention reduces the time in the blowing step by cooling the blown article faster so that it becomes stiff enough, in a shorter time, for removal from the blowing mold. Cooling liquid circulates through chambers in the blow mold and this cools the blown article which contacts with the surface of the blow mold cavity.
Such cooling of the plastic is entirely from the outside surface of the plastic. Since plastic materials are not particularly good conductors of heat, the cooling can be performed faster if the plastic is cooled from the inside as well as from the outside. This invention cools the plastic of the blown article from the inside by circulating a mist in the space enclosed by the blown article. The mist consists of small globules of liquid carried by a gas. The liquid in the gas stream increases the heat conducting capacity of the cooling fluid because the specific heat of the water globules is higher than that of the gas and thus increases the specific heat of the mist as compared to that of a dry gas.
To the extent that liquid globules contact with the inside surface of the plastic and evaporte, the cooling is greatly increased by the latent heat of vaporization of the liquid being absorbed from the surface of the plastic. In the case of water, only one BTU need be added to a pound of water to raise the temperature of the water 1.degree. F; but 970 BTU must be added to the water to change the water from liquid to vapor at the same temperature. Thus evaporation of water within the space enclosed by the blown article greatly increases the rate at which heat can be extracted from the plastic with resulting decrease in temperature of the plastic.
This invention also requires special construction of a core rod. Ordinary core rods have provision for blowing air from the core rod to expand a parison on the core rod into contact with the sides of a blow mold cavity. The amount of air entering the blown article is equal to the inside volume of the blown article. While some of this air escapes from within the blown article when the pressure decreases, there is no actual circulation of air or other cooling fluid through the article in the blow mold.
The core rod construction of this invention provides for the inlet and exhaust of the mist at the same time so that there is a steady circulation of cooling fluid through the blown article to provide inside cooling. In the preferred construction, a sleeve on the outside of the core rod is movable lengthwise of the core rod to cover up passage outlets from the core rod while the core rod is in an injection mold. This prevents these passages from being clogged by plastic when the parison is applied to the core rod.
The invention also provides an improved method for cooling a blown article from the outside and inside simultaneously.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.